Medical devices meant for temporary or semi-permanent implant are often made from stainless steel. Stainless steel is strong, has a great deal of load bearing capability, is reasonably inert in the body, does not dissolve in bodily fluids, and is durable, lasting for many years, if not decades. Long lasting medical implants, however, are not always desirable. Many devices for fixing bones become problematic once the bone has healed, requiring removal by means of subsequent surgery. Similarly, short term devices such as tissue staples have to be removed after the tissue has healed, which limits their use internally.
Attempts to generate biodegradable materials have traditionally focused on polymeric compositions. One example is described in U.S. Pat. No. 5,932,459, which is directed to a biodegradable amphiphilic polymer. Another example is described in U.S. Pat. No. 6,368,356, which is directed to biodegradable polymeric hydrogels for use in medical devices. Biodegradable materials for use in bone fixation have been described in U.S. Pat. No. 5,425,769, which is directed to CaSO4 fibrous collagen mixtures. And U.S. Pat. No. 7,268,205 describes the use of biodegradable polyhydroxyalkanoates in making bone fasteners such as screws. However, none of the biodegradable polymeric materials developed to date have demonstrated sufficient strength to perform suitably when substantial loads must be carried by the material, when the material is required to plastically deform during implantation, or when any of the other native characteristic of metal are required from the material. For example, the polyhydroxyalkanoate compositions described in U.S. Pat. No. 7,268,205 do not have sufficient strength on their own to bear weight and must be augmented by temporary fixation of bone segments. In addition, biodegradable polymeric materials tend to lose strength far more quickly than they degrade, because the portions of the material under stress tend to be more reactive, causing preferential dissolution and breakdown at load-bearing regions.
Metals, particularly steels, are thus preferred for the construction of many medical implants. The performance characteristics of steel closely match the mechanical requirements of many load bearing medical devices. Although ordinary steel compounds, unlike stainless steel, will degrade in biological fluids, they are not suitable for use in biodegradable implantable medical devices. This is because ordinary steels do not degrade in a predictable fashion, as one molecule or group of molecules at a time, which can be easily disposed of by the body. Rather, because of their large-grain structures, ordinary steels tend to break down by first degrading at grain boundaries, causing fissures and separations in the medical device, followed by rapid loss of strength and integrity and particulation. Particulation of the medical device is extremely dangerous because it allows small pieces of the device to leave the area of implantation and become lodged in other tissues, where they can cause serious injury including organ failure, heart attack and stroke. The use of ordinary steels in implantable medical devices is also complicated by the fact that ordinary steels typically contain alloying elements that are toxic when released in the body.
There remains a need in the field to develop additional implantable medical devices that have desirable characteristics associated with steel but which are also biodegradable.